Coaxial filament transformers for pulse circuits



Jan. 1, 1963 3,071,734

COAXIAL FILAMENT TRANSFORMERS FOR PULSE CIRCUITS Filed Feb. 14, 1961 D.O. KIPPENHAN 2 Sheets-Sheet 1 INVENTOR DEAN O. KIPPENHAN ATTORNEY Jan.1, 1963 D. o. KIPPENHAN 3,071,734

COAXIAL FILAMENT TRANSFORMERS FOR PULSE CIRCUITS Filed Feb. 14, 1961 2Sheets-Sheet 2 1 N V ENTOR mOEDOw mmJDm ozEE DEAN O. KIPPENHAN ATTORNEYUnited States Patent ()filice ifillfllit i atentecl Jan. ll, 19633,071,734 COAXIAL FHLAMENT TRANSFORWER FOR PULSE ClRCUiTfi Dean 6).Kippenhan, Castro Valley, Califi, assignor to the United States ofAmerica as represented by the United States Atomic Energy CommissionFiled Feb. 14, 1961, Ser. No. 89,326 12 Claims. Cl. 328270) The presentinvention relates to filament power supplies for electron tubes in pulsecircuits and more particularly to means for isolating a filament powersupply from the high voltage, high speed pulses in such circuits wherebythe shape of such pulses may be more accurately controlled andmaintained.

In high voltage, high speed pulsing circuits it is highly desirable thatmeans he provided to prevent unwanted interaction between the filamentpower supply and the pulse load portion of the circuit. Such means willaid in preserving the desired waveshape and also provide for keepinghigh voltage pulses out of the filament power supply. The severity ofthis problem varies directly with the pulse voltage, indirectly with thepulse duration and directly with the rise time of the pulse. The veryhigh power pulse characteristics of recent developments, such ascontrolled thermonuclear reaction containment devices, have greatlyincreased the problem and have tended to result in undue complication ofthe circuitry.

To meet the problem discussed above, previously used high voltage, highspeed pulsing circuits have depended largely on filtering circuits tomaintain desired pulse shapes and keep pulses out of the filament powersupply. With the use of high voltages and greater speeds this filteringhas become both cumbersome, expensive and often inefiective. From thesestandpoints previously used methods of isolating the filament powersupply equipment are not satisfactory and may not produce pulses of thedesired form. Another method which has been used in the past hasconsisted of a switching arrangement which removes the filament powersupply from the circuit prior to pulsing. This arrangement is alsoexpensive and has not been very satisfactory in operation due torepetition rate limitations.

The present invention, by making use of coaxial cable and balancedcoaxial load circuits, balances out the pulse eifects from the filamentpower supply and efiectively removes the filament power supply frominfluencing the pulse shape. In this manner the requirement for theunsatisfactory and expensive filter or switching circuits formerly usedis eliminated and a circuit which will produce properly spaced pulses athigh output power is achieved.

The invention provides for the foregoing result by means of a filamenttransformer having a coaxial cable as a secondary winding. The outerconductor of such cable is broken at the midpoint so that no transformersecondary current is carried in the outer conductor. Current to thefilament of the associated tube is carried by the inner conductor whichis continuous throughout the coaxial cable. As will hereinafter bedescribed in detail the foregoing structure allows the coaxial cable tobe used as a component of the tube pulse circuitry inasmuch as currentswithin the cable, other than the filament current, may be balanced.

Therefore it is an object of this invention to provide a meanspreventing unwanted interaction between the filament power supply andthe pulse load circuit of a pulse generator.

it is an object of the present invention to provide. a circuit whichwill preserve desired output wave shapes in a high voltage highrepetition rate pulsing circuit.

It is another object of this invention to provide a means for supplyinga filament through balanced coaxial lines in an arrangement wherebypulse effects of associated circuitry are blocked from the filamentsupply.

A further object of the present invention is to provide a filament powersupply with which a high power pulse generator may be made to producepulses having very short rise times.

Another object of the invention is to eliminate the need for cumbersomeand expensive filter equipment for the purpose of maintaining desiredoutput pulse shapes in a high wattage high repetition rate pulsingcircuit.

A further object of this invention is the elimination of the need forusing a switching arrangement to remove the filament supply from apulsing circuit prior to and during pulsing. I

It is also an object of this invention to provide a filament circuit fora pulse generator, portions of which filament circuit may be used totransmit pulses without such pulses affecting the filament power supply.

Other objects and advantages of this invention will become obvious tothose skilled in the art upon consideration of the following descriptionand accompanying drawing, of which:

FIGURE 1 is a partially cut away, partially schematic view of theinvention as applied to a pulsing circuit designed to deliver positivesquare wave pulses to a load.

FIGURE 2 is a cross sectional view of a modified form of a crossconnector utilized in the invention, and

FIGURE 3 is a partially cutaway, partially schematic, view of theinvention applied to a pulse circuit designed to deliver negative pulsesto a load.

Considering now a first embodiment of the invention as illustrated inFIGURE 1, the pulse switching structure 11 is partially contained withina cylindrically shaped metallic tank 12 provided with flanges 13 and 14at the ends. Circular metallic end plates 16 and 18 are secured againstthe ends of tank 11 by means of bolts 17'. Mounted coaxially within thetank 12 is an electron tube 21 which may be a type 5C22 thyratron, forinstance, having anode 22, control grid 23, cathode 24 and heaterelement 26. The tube 21 is so mounted in the tank 12 that the base isnearer the end plate 16 and the anode terminal is nearer the end plate18. One suitable physical arrangement for the coaxial mounting of thetube 21 within tank 12 is disclosed in US. Patent No. 2,776,368, issuedJanuary 1, 1957 to Harvey M. Owren, Vernon L. Smith and David R. Barnumand entitled Coaxial Tube Coupling. To more clearly illustrate thepresent invention, tube 21 and certain associated components are shownschematically in FIGURE 1, and reference should be made to the specifiedpatent for suitable physical structure. Anode 22 is connected through aresistor 27 and coaxial cable 28 to a source of positive high voltage29, the coaxial cable 28 being secured to the end plate 18 by means of asuitable cable connector 31. For high voltage usage, for instance at 16kilovolts, the coaxial cable connector 31 comprises a metal tube 32brazed or otherwise secured into an opening in end plate 18. The innerconductor insulation 33 of the coaxial cable passes through the tube 32with a fairly snug fit and protrudes a short distance into the tank 12with inner conductor 34 protruding from the end thereof. An exposedsection of outer braid conductor 36 covers the outer end of tube 32 andis held thereon by a clamp 37. All of the cable connectors used in thisinvention are preferably of the described type for the high voltageusage herein considered, however it is evident that where lower voltagesare used a standard type of cable connector may be employed.

The braid contact with tube 32 provides an electrical connection throughend plate 18 to the tank 12 which tank is provided with an electricalground 38 placing the entire enclosure at ground potential. For thepurposes of pulse generation anode 22 is also connected to both ends ofa length of coaxial cable 39- Which may be 60 feet in length, forinstance, or if desired a pulse forming network of the proper constantsmay be used instead. This coaxial cable 3 is secured to the end plate 13by means of coaxial cable connectors 41 and 42 which ground the outerconductor of the cable to the plate 18, the inner conductor remaininginsulated therefrom. Coaxial cable 39, serving as a charging line, maybe used in one piece as shown or may be cut at the exact midpointforming two 30 foot lengths of cable each of which is unterminated atthe end remote from the coaxial enclosure 11. The double endedconnection for cable 39 is made for the purpose of matching theimpedance of the anode circuit to the cathode circuit, which will bedescribed later, and to obtain the desired pulse current.

Control grid 23 of the tube 21 is connected through a resistor 43 to thecathode 2 and through a second resistor 44- and a capacitor 46 to acoaxial cable 47. A first end of cable 57 is secured to the end plate 16by means of coaxial connector 48 and the other end of coaxial cable 47is connected to a firing pulse source 49. The outer conductor of thecable 47 is grounded to end plate 16 through connector 48. The cathode24- is further connected to one side of the heater element 26 acrosswhich element a capacitor 51 is connected to bypass pulses from theheater element 26.

The side of the heater element 26 to which the cathode is connected isfurther connected to an end of the inner conductor of a coaxial cable52. The outer conductor of the coaxial cable 52 is electrically andmechanically connected to the end plate 16 by means of a connector 53which permits the inner conductor of the cable 52 to pass therethrough.Similarly the other side of heater element 26 is connected to the innerconductor of another coaxial cable 54, of exactly the same length ascable 52, the outer conductor of cable 54 being electrically andmechanically secured to end plate 16 by means of a coaxial connector 56.The ends of coaxial cables 52 and 54 remote from end plate 16 are bothterminated in a novel coaxial cross connector 57 which will hereinafterbe described.

Cables 52 and 54 must be of sufiicient length that a number of turns ofone of the cables, cable 54 in this instance, may be utilized assecondary turns on the core 58 of a heater supply transformer 59. Thenumber of turns of cable 54 on the core 58 is selected to have theproper ratio with the turns of a primary winding 61 thereon to providethe proper heater voltage when the primary winding is connected toelectrical power source 62.

Considering now the detailed structure of the cross connector 57, suchconnector is formed with four mutually perpendicular arms. An innerconductor 63 and outer conductor 64 of the first arm are connected tothe inner and outer conductors respectively of coaxial cable 52. Innerconductor 66 and outer conductor 67 of a second adjacent arm of thecross connector are connected to load 70 with which the pulse current isused. Inner conductor 63 and outer conductor 6% of a third arm of thecross connector are connected to the inner and outer conductorsrespectively of coaxial cable 54. Inner conductor '71 and outerconductor 72 of the fourth arm of the cross connector are connected to atermination 75 equaling the pulse load in impedance, in order to balancethe circuit. The inner conductors 63, 66, 63 and 71 of the crossconnector 57 are all connected in a common junction 73. The outerconductors 64 and 67 of the first two arms are joined togetherelectrically and mechanically and the outer conductors 6% and 72 of thethird and fourth arms are joined together in a similar manner, the twosets of joined outer conductors being insulated from each otherelectrically by a gap In operation, since the inner conductors of thecrossconnector 57 are joined at the junction 73,, the inner conductorsof the coaxial cables 52 and 54 form a closed loop with the heater 26thus providing heater current to the tube filament 26. Such currentarises through the voltage developed in the turns of cable 54 which formthe secondary winding of transformer 59. The same voltage is developedin the outer conductor of cable 54, but the open circuit at gap 86 ofcoaxial cross connector 57 blocks the flow of current in this path andcauses the heater current to flow only in the inner conductors of thecables.

A high positive voltage from source 29 is applied to the inner conductorof coaxial cable 28 which, through resistor 27, charges the coaxialcable 39 to the positive voltage applied. During this period tube 2.1 isin a nonconducting condition. After the coaxial cable 39 is fullycharged, a positive voltage pulse is delivered to the grid 23 of tube 21through coaxial cable 4-7 and through capacitor 4-6 and resistor 44.Such pulse causes the tube to become conductive and the charged coaxialcable 39 discharges through the tube. Such discharge is through the tubecathode, and approximately half of the discharge passes through coaxialcable 52, and inner conductors 66 and 71 of cross-connector 57 to theload and the load balancing impedance respectively. The other half ofthe discharge passes through capacitor 51, which capacitor eifectivelyshort circuits heater 26 to high frequency cur rents, and thence throughcoaxial cable 54 to the inner conductors 66 and 71 and to the load 70and the load matching impedance '75. Thus the two branches of thecurrent reach junction 73 and conductors 66 and 71 simultaneouslythrough the two described parallel paths and are transmitted to the loadand the load balancing impedance. The pulse in passing through cable 54is influenced only by the coaxial cable 54 and sees no inductance fromthe turns Wound on core 58. The portion of the current passing throughthe load takes a return path through the outer shells 67 and 64 and theouter conductor of the coaxial cable 52 and through coaxial connector 53and end plate 16 to ground. The currents pass ing through the loadmatching impedance pass outer shells 72 and 69, the outer conductor ofcoaxial cable 54, the shell of coaxial connector 56 and end plate 16 toground.

Since the pulse currents in the inner and outer conductors of coaxialcable 54- are equal and opposite in direction the discharge pulse has noeifect on the core 58 and the heater supply is thus effectively isolatedfrom the pulses generated in the circuit. The pulses delivered to theload in this instance are positive and pulses of high power shortduration and having square waveshape may be successfully developed.

By a modification of the circuit the load matching impedance may beeliminated and the entire pulse output taken through the load. As shownin FIGURE 2, this may be accomplished by removing the fourth arm ofcross-connector 57, which consists of the inner conductor 71 and outerconductor 72, which leads to the load balancing impedance. A smallcapacitor 74 may then be connected from the outer shell '64- of thefirst arm of cross-connector 57 to the outer shell 69 of the third arm.This capacitor may be so selected that the resistance to the lowfrequency heater current may be quite high for instance 2,000 to 3,000ohms, and the voltage developed across the capacitor is very low ascompared to output pulse voltage. This connection maintains circuitbalance so that the pulse currents flowing in the inner and outerconductors of coaxial cable 54 are equal and opposite in directionthereby protecting transformer 59 from the effects of high voltagepulses.

For the switching of negative pulses only minor changes in the circuitare necessary. A second embodiment of the invention, for producingnegative pulses, is shown in FIG- URE 3 the design thereof being similarto the embodiment of FIGURE 1 except as will now be described. In thisembodiment the high voltage lead 28, instead of providing a highpositive voltage through resistor 27 to the anode 22, provides a voltagefrom a negative high voltage source 76 to the cathode 24 through aresistor 27. The anode 22 of tube 21 is connected through a coaxialcable 77 to a pulse load 78 through a Coaxial cable connector 79.Similarly the anode 22 is connected to a load balancing impedance 83through a second coaxial cable 81 and connector 82. In this embodimentthe coaxial crossconnector 57 of FIGURE 1 is omitted from the circuitand the two filament supply coaxial cables 52 and 54 together form thecharging line, as well as the secondary winding of transformer 59. Inthis circuit as in the positive pulse switching circuit the two coaxialcables 52 and 54 are made identical in length, for instance 30 feeteach, and the inner conductors of these two cables are connectedtogether at the ends remote from the coaxial con nectors 53 and 56. Theouter conductors are left unconnected or open at the ends remote fromthe coaxial connectors 53 and 56 providing a break 84- in the outerconductor of the charging line at its midpoint. A suflicient number ofturns of either coaxial cable 52 or coaxial cable 54- or a combinationof the two is wound on the core 58 of transformer 59 to provide therequired voltage to heater element 26 as described for the firstembodiment. All other elements of the circuit are substantially the sameas for the previously described positive pulse generating circuit.

In operation the negative pulse generating circuit is turned on byconnecting a source of electrical power 62, for instance 115 volts, 60cycles, to the primary coil 61 of transformer 59. This generates voltagein the inner conductors of cables 52 and 54 to provide the currentnecessary for the heater 26 as heretofore described. 'As discussed, thesame heater voltage is developed in the outer conductors of cables 52and 54 but since the outer conductors are not connected together theheater voltage appears across the break 84 in the outer conductorcircuit but no current flows in this circuit as a result of thisvoltage. A high negative voltage is applied through resistor 27 tocathode 24 and to the coaxial cables 52 and 54 thus charging this linenegatively. During this period the tube 21 is in a non-conductingcondition.

After the cathode is. heated and the charging line completely charged afiring signal, for instance a positive voltage pulse, may be introducedto the control grid 23 of tube 21 through capacitor 46 and resistor 44causing the tube to become conductive and discharging the charging linecircuit through the tube, and thence through the coaxial cable 77 andthe load 78 and through the parallel circuit formed by coaxial cable 81and load balancing impedance 83. Since the charging line and cathode 24are at a high negative voltage and the anode 22 is at ground potentialduring cut off of tube 21, the firing of the tube sharply reduces theinternal resistance thus suddenly lowering the voltage of the anode 22and producing the negative pulse in the load circuit and in the parallelload matching circuit. The pulse of current discharging the coaxialcables 52 and 54 is capacitative in nature between inner and outerconductors and therefore the pulse current in the inner conductor ofeach of these cables will be equal to and opposite in direction to thepulse current in the outer conductor. The voltage wave of the dischargepulse travels along the two cables 52 and 54 and reaches the commonpoint of juncture simultaneously thus balancing out and causing nocurrent to flow around the loop. Therefore currents will balance in bothdirections whereby pulse shapes will be preserved and pulse currentswill be kept out of the power supply unit.

In the negative pulse switching circuit as in the first embodiment ofthe invention it is possible to eliminate the load matching impedance bya proper balance of impedances. In such an arrangement the impedance ofa single coaxial cable terminated in a load may be designed to equal theimpedance of the parallel combination consisting of coaxial cables 77and 81 connected to the load 78 and load balancing impedance 83respectively. The

5 single cable with load may then be substituted for the parallelcombination.

Although the present invention has been disclosed with respect to alimited number of exemplary embodiments, it will be evident to thoseskilled in the art that many variations are possible within the spiritand scope of the invention. Therefore it is not intended to limit theinvention except as defined by the following claims.

What is claimed is:

1. In a pulse circuit of the class utilizing an electron tube with afilament therein, a filament heater circuit for said tube comprising, incombination, a transformer having a core with a primary winding thereon,means for connecting said primary winding to a source of electric power;and a coaxial cable having an inner and an outer conductor, a portion ofsaid cable being turned around said core of said transformer to form asecondary winding thereon, said outer conductor of said cable beingdiscontinuous at a central portion thereof and said inner connector ofsaid cable being continuous and. being connected to said filament tosupply heater current thereto.

2. In a pulse circuit of the class employing an electron tube having afilament therein, a filament heater circuit for said tube comprising, incombination, a transformer having a core with a primary winding thereon,means for connecting said primary winding to a source of electric power,and a coaxial cable having an inner and an outer conductor, a portion ofsaid cable being wound on said core to form a secondary winding thereon,said cable having a break in the outer conductor at the midpoint of thelength thereof, the inner conductor of said cable being continuous andconnected at the ends thereof to said filament, to supply heater currentthereto.

3. In a pulse circuit of the class employing an electron tube providedwith a filament, a filament heater circuit for said tube comprising, incombination, a transformer having a core and a primary winding thereon,means for energizing said primary winding, a coaxial cable having aninner and an outer conductor, a portion of said cable being turnedaround said core of said transformer to form a secondary windingthereon, said outer conductor of said cable being electricallydiscontinuous at an intermediate point thereof and said inner conductorof said cable being continuous throughout and having ends connected tosaid filament to supply heater current thereto, and a capacitorconnected across said ends of said inner conductor.

4. In a pulse circuit of the class having an electron tube with acathode and cathode heater therein, a cathode heater circuit for saidtube comprising, in combination, a transformer having a core with aprimary winding thereon, means for connecting said primary winding to asource of alternating current, and a coaxial cable having an inner andan outer conductor, a portion of said cable being wound around said coreof said transformer to form a secondary winding thereon, said outerconductor of said cable having a gap at the midpoint thereof and saidinner conductor of said cable being continuous and being connected tosaid cathode and connected in series with said cathode heater to supplyheater current thereto.

5. In a pulse circuit of the class having an electron tube with afilament therein, a filament heater circuit for said tube comprising, incombination, a transformer having a core with a primary winding thereon,means for connecting said primary winding to a source of electric power,a coaxial cable having an inner and an outer condoctor, a portion ofsaid cable being wound around said core of said transformer to form asecondary winding thereon, said outer conductor of said cable beingelectrically discontinuous at a central point thereof to separate saidouter conductor into a first and second component of substantially equallengths and said inner conductor of said cable being electricallycontinuous and being connected to said filament to supply heater currentthereto, and means for connecting a load across said inner conductor ofsaid cable and said first component of said outer conductor thereof,saidconnection being made at said central point of said cable.

6. A pulse circuit as described in claim 5 and comprising the furthercombination of impedance matching means connected between said innerconductor of said cable and said second component of said outerconductor of said cable, said connection to said impedance matchingmeans being at said central point of said cable.

7. An electronic pulse circuit comprising, in combination, an electrontube with anode, control grid and filament therein, a charging lineconnected to said anode, means for providing firing signals to saidcontrol grid, a transformer having a core and having a primary windingthereon, means for connecting said primary winding to a source ofelectric power, a coaxial cable having a plurality of turns on said coreto form a secondary thereon, the outer conductor of said cable beingopen at the midpoint of said cable and the inner conductor beingcontinuous and connected at both ends to said filament to furnishheating current thereto, means for connecting a load between said outerand inner conductors of said cable at said midpoint thereof, and meansfor connecting a load balancing impedance between said outer and innerconductors.

8. An electronic pulse circuit comprising, in combination, an electrontube with control grid, anode and fila ment therein, said control gridbeing connected to a source of trigger pulses, a charging line connectedto said anode, a transformer having a core with a primary windingthereon, means for connecting said primary winding to a source ofelectrical power, a coaxial cable having inner and outer conductors,said outer conductor having a break at the midpoint and said innerconductor being continuous and being connected at both ends to saidfilament, said cable having a plurality of turns wound on said core as asecondary winding thereon whereby heater current is provided for saidfilament, means for connecting a load to said cable at said break insaid outer conductor said load being connectable from the innerconductor to the outer conductor of said cable at a first side of saidbreak and means for connecting a load matching impedance between saidinner conductor and said outer conductor of said cable at the secondside of said break therein.

9. An electronic pulse switching circuit comprising, in combination, anelectron tube with control grid, anode, cathode and heater therein,means for transmitting trig- ;ger pulses to said control grid, a firstcoaxial cable connected to said anode and constituting a charging linetherefor, a high voltage source connected to said first coaxial cableand said anode, a capacitor connected across said heater, a transformerhaving a core with a primary winding thereon, a second coaxial cablehaving inner and outer conductors, said outer conductor having a gap atthe midpoint dividing said outer conductor into electrically separatefirst and second segments and said inner conductor being continuous andseries connected to said heater, a plurality of turns of said cablebeing wound on said core as a secondary winding whereby current isprovided for said heater, a third coaxial cable connected with saidsecond cable at said gap therein for coupling a load to said circuit,said third cable having an outer conductor connected with said firstsegment of said outer conductor of said second cable and having an innerconductor connected to said inner conductor of said second cable, and afourth coaxial cable connected with said second cable at said gaptherein for coupling a load balancing impedance to said circuit, saidfourth cable having an outer conductor connected with said secondsegment of said outer conductor of said second cable and having an innerconductor connected to said inner conductor of said second cable.

10. An electronic pulse circuit comprising, in combination, an electrontube with control grid, anode and filament therein, means for connectingsaid control grid to a source of firing pulses, a charging lineconnected to said anode, a high voltage supply connected to saidcharging line and said anode, a transformer having a core and having aprimary winding thereon, a coaxial cable having inner and outerconductors with said outer conductor having a gap at the midpoint toform first and second electrically separate segments of said outerconductor, said inner conductor being continuous through said gap andconnected at both ends to said filament, a plurality of turns of saidcable being wound on said core as a secondary winding thereon wherebyheater current is provided for said filament, means for connecting aload to said coaxial cable at said gap in said outer conductor saidconnection being from said inner conductor to the first segment of saidouter conductor and a capacitor connected across said gap in said outerconductor.

11. An electronic pulse circuit comprising, in combination, an electrontube with control grid, anode and filament therein, said control gridhaving means for connection to a source of trigger pulses, means forconnecting a load to said anode, a high voltage source coupled to saidcathode, a transformer having a core with a primary Winding thereon anda coaxial cable having inner and outer conductors, said inner conductorbeing continuous and having the two ends thereof connected to saidfilament and said outer conductor having a break at the midpoint andbeing grounded at both ends whereby each half of said coaxial cable actsas a charging line, a plurality of turns of said coaxial cable beingwound on said core providing a secondary winding to supply heatercurrent to said filament.

12. An electronic pulse switching circuit comprising, in combination, anelectron tube with a control grid and an anode and a connected cathodeand heater therein and having means for connecting said control grid toa source of trigger pulses, a voltage supply connected to said cathode,a load connected to said anode, a capacitor connected across saidheater, a transformer having a core with a primary winding thereon andhaving means for connecting said winding to a source of alternatingcurrent, and a coaxial cable having inner and outer conductors, saidouter conductor having a discontinuity at the midpoint whereby each halfof said coaxial cable acts as a charging line, said inner conductor ofsaid cable being continuous through said midpoint and being connected atboth ends to said heater, a plurality of turns of 7 said coaxial cablebeing wound on said core as a secondary winding thereon whereby currentis provided for said heater.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A PULSE CIRCUIT OF THE CLASS UTILIZING AN ELECTRON TUBE WITH AFILAMENT THEREIN, A FILAMENT HEATER CIRCUIT FOR SAID TUBE COMPRISING, INCOMBINATION, A TRANSFORMER HAVING A CORE WITH A PRIMARY WINDING THEREON,MEANS FOR CONNECTING SAID PRIMARY WINDING TO A SOURCE OF ELECTRIC POWER;AND A COAXIAL CABLE HAVING AN INNER AND AN OUTER CONDUCTOR, A PORTION OFSAID CABLE BEING TURNED AROUND SAID CORE OF SAID TRANSFORMER TO FORM ASECONDARY WINDING THEREON, SAID OUTER CONDUCTOR OF SAID CABLE BEINGDISCONTINUOUS AT A CENTRAL PORTION THEREOF AND SAID INNER CONNECTOR OFSAID CABLE BEING CONTINUOUS AND BEING CONNECTED TO SAID FILAMENT TOSUPPLY HEATER CURRENT THERETO.